The in vitro serological detection of H-Y (male) antigen on the surface of mouse spermatozoa and male epidermal cells, with the cytotoxic test, provides the means for a closer evaluation of humoral versus cellular immune factors involved in H-Y incompatibility, and of other questions about the H-Y antigenic system. The importance of the H-Y antigen in developmental biology has been reviewed by Gasser and Silvers (Adv. Immunol. 15: 215, 1972). Our aim is to: (1) Develop procedures for the immunoselection of female embryos prior to implantation; (2) Determine to what extent there is polymorphism for H-Y within the species (mouse); (3) Determine what the role of immunologic enhancement may be in the (a) long-term survival of male grafts on certain strains of mice which nevertheless (as we now know) produce cytotoxic H-Y antibody, and (b) in the specific tolerance to the H-Y antigen exhibited by multiparous females; (4) Determine the relationship between sperm surface antigens and tumor associated antigens, and (5) Determine whether or not antigenic representation on the sperm surface is coded by its haploid genotype. In a broader context, in as much as H-Y incompatibility is now seen to be a system in which graft rejection may not occur despite an immune response (antibody production), it provides a parallel with the growth of antigenic tumors in hosts that produce antibody and therefore has relevance to cancer as well as to transplantation biology.